1. Field of the Invention
The present invention relates to fiber reinforced thermoplastic materials and more particularly to a technique to reduce the stresses therein.
2. Description of Related Art
Thermoplastics reinforced with continuous high strength, high modulus fibers (such as graphite and glass) are known to exhibit enhanced structural properties and manufacturing cost advantages over more conventional fiber-reinforced thermoset composites. In addition, these materials have stiffness-to-weight and strength-to-weight properties which are superior to many metals.
However, as with other composite laminates, thermoplastic composite laminates are prone to matrix cracking and interlaminar failure due in part to what is referred to in the art as the `Free Edge Effect`. Interlaminar failure is characterized by progressive delamination of the plies of a laminated composite structure leading to a loss of stiffness and strength of the structure. Interlaminar failure is one of the most prevalent and serious failure modes of fiber-reinforced composite structures.
The `Free Edge Effect` is a term given to the increased and highly localized stresses which occur in the vicinity of a free edge as a result of the edge's `traction-free` condition. By `traction free`, it is meant that no forces can be transferred beyond the free edge and, therefore, all in-plane stresses within the material must be balanced to zero at the very limits of the free edge, that is, usually within a distance equal to the thickness of the free edge itself.
The manner in which this occurs is very complex but, simply stated, certain internal stress components become highly intensified in order to balance the in-plane stresses at the free edge. It is these intensified stress components which most often cause the interlaminar failure of the laminate.
The `Free Edge Effect` is believed to arise in part from residual stresses that exist in the laminate. Each of the laminations or plies has a temperature expansion coefficient which is relatively low in the direction of the reinforcing fibers and relatively high in the direction transverse to the reinforcing fibers. When the plies are bonded together, the fibers may be oriented at different angles to provide strength in certain directions, for example at 45, 90, 135 degrees and so on. When the laminate cools from its temperature of manufacture, residual stresses develop both within and between the plies due to the differential thermal shrinkage occurring from one ply to the next.
Since the intensified residual stresses which arise from the `Free Edge Effect` contribute so strongly to interlaminar failure modes, it is not surprising that a great deal of study has been done to find methods of preventing these failure modes.
There appear to be three conventional solutions to this problem, namely:
1) finding stronger matrix materials to resist the `Free Edge Effect`; PA1 2) using some form of `three dimensional reinforcement` (such as "stitching") along the free edge, to inhibit the `Free Edge Effect` from inducing interlaminar failure; and PA1 3) using layers of relatively tougher material between the plies (referred to in the art as `adhesive interlayers`) to provide a tough `buffer` layer which resists the high residual stresses at the free edge. PA1 i) providing a fiber reinforced thermoplastic composite laminate having at least one free edge; PA1 ii) heating a region adjacent the free edge for a time period sufficient to form a localized melt zone of the thermoplastic in the region and under a sufficient pressure to prevent delamination of the composite and thereafter; PA1 iii) cooling the region for a time period and under a pressure sufficient to solidity the region and to prevent delamination thereof. PA1 i) an improvement in the properties of the matrix; PA1 ii) a change in the chemistry of the thermoplastic matrix; or PA1 iii) the provision of inter-layers or stitching are not necessary to achieve this increase in strength.
While these solutions provide reasonable results, they tend to be expensive and the use of the reinforcement and the interlayers is cumbersome. It is for these reasons that there remains a need for a technique to reduce the `Free Edge Effect`, rather than merely to prevent failure due to it. It is an object therefore of the present invention to provide just such a technique.